In ELF (extremely low frequency) and VLF (very low frequency) communications systems, it is normal practice to employ a transmitting antenna for launching a vertical electrical field (E-field) into the cavity or space between the earth's surface and the ionosphere to be thereupon detected by receivers within this space. Generally speaking, in the VLF band, such an antenna may take the form of a vertical loop or vertical mast or , alternatively, in the ELF/VLF band, a horizontal wire located within or close to the earth's surface. Vertical transmitting loops or masts, while suitable for short-range applications of less than 50 miles, are impractical because of their low radiation efficiency for long distance communications, for example, over a range of several thousand miles. In this latter instance, the vertical loop or mast (at ELF) must be impractically tall, for example, several thousand feet high, to achieve efficient long-range operation.
The use of horizontal wire antennas for E-field transmission also has disadvantages and drawbacks. This type of antenna, whether located above ground (and uninsulated) or below ground (and insulated), requires a low-conductivity subsurface for efficient operation, as well as requiring sizable grounding networks at its two ends. These grounding networks, which generally include a few miles of bare wire buried into the earth several feet below the earth's surface (e.g., 5-6 feet), are expensive and difficult to install and to balance electrically. The grounding networks can also create potential safely hazards when used at high current levels, for example, 300 amperes. Further, because of ohmic resistance to current flow through the ground (between the ends of the wire antenna), the efficiency of the antenna is degraded and results in appreciable power losses. If the wire is formed into a loop with a horizontal orientation, the resulting electric field is principally horizontally-oriented and the effective transmitting range of the loop is less than 50 miles and therefore unsuitable for long-range operation.